Soil pollution poses a serious threat to the ecological environment, food safety and human health. The soil pollution caused by heavy metal pollutants has been particularly prominent in recent years, and the pollution of soil contaminated with chromium is especially outstanding.
China has nearly 70 major chromium slag contaminated sites, together with a large number of enterprises engaging with electroplating, leather processing and other business, it causes serious chromium pollution of soil, threatens or endangers groundwater and drinking water sources. In addition, municipal solid wastes (sludge, fly ash, garbage) and phosphate fertilizers contain chromium. Due to the agricultural reuse of these solid wastes having certain fertility, the chromium content in the soil is higher than the environmental background value and becomes one of the sources of chromium pollution.
The treatment and renovation technology of chromium-contaminated sites is the focus and difficult problem of environmental protection and technology research at home and abroad. Chromium exists in the nature world mainly with two stable valence states, namely hexavalent chromium (VI) and trivalent chromium (III). Trivalent chromium is mainly presented in the ionic form of Cr3+, which has low activity and toxicity; while the hexavalent chromium is mainly presented in the ionic forms of HCrO4− and CrO42−, both are easily soluble in water and have high activity and strong toxicity.
The current renovation technologies of chrome-contaminated soil may be classified into two types, the first type of renovation technology focuses on modifying the existence form of chromium in the soil, reducing hexavalent chromium to be trivalent chromium, bringing down its migration ability and bioavailability of hexavalent chromium in the environment, thereby reducing the hazard of the chromium pollutants; the second type of renovation technology intends to completely remove chromium from the chrome-contaminated soil. At present, the remediation technology for chrome-contaminated soil mainly comprises bioremediation method, physical restoration method, physical and chemical restoration method, and chemical renovation method.
The bioremediation method includes phytoremediation and microbial remediation. The microbial remediation uses indigenous microorganisms in the original soil or supplements the domesticated and efficient microorganisms to the polluted environment. The hexavalent chromium is reduced into the trivalent chromium by a biological reduction reaction under the optimized operating conditions, thereby renovate the contaminated soil. The bioremediation method has the advantages of desirable renovation effect, low cost and small secondary pollution, but the renovation cycle is very long.
The physical restoration method uses relatively classic measures for controlling chromium pollution in soil, the method mainly includes measures such as earth removed from some other place to improve the local soil, soil replacement and deep tillage for turning the soil. By means of mixing the polluted soil with removed earth, replaced soil and the soil turned by deep tillage, the content of chromium in the polluted soil may be reduced, thereby bring down the toxicity of chromium to soil and plant systems. The physical restoration method has the advantages that the method is simple, the spent time is short, and the renovation is complete and stable, but its implementation has many disadvantages, such as it requires a large amount of earthwork, the investment and cost is high, the contaminated soil still exists; in addition, it destroys the soil structure, causes a decline of the soil fertility, and the transferred soil shall be stacked or processed. However, the removed soil or soil replacement is still a practical and effective method for treating a small area of seriously polluted soil.
Physical and chemical restoration method includes the processes of electrokinetic remediation, electrothermal restoration and soil leaching. The electrokinetic remediation relates to inserting the cathode and the anode into the soil, applying the electrodes with direct current, and moving Cr6+ to the cathode under the action of the electric field, then carrying out centralized treatment. Electrothermal restoration uses high-frequency voltage to generate magnetic waves, which heats the soil and separates the chromium from the soil to fulfill the purpose of restoration. The soil leaching uses an eluent to transfer chromium from the soil solid phase to the soil liquid phase, and performs further treatment on the chromium-containing water. Although the physical and chemical restoration method has many advantages, it suffers from insurmountable shortcomings such as the energy consumption is relatively high, the renovated soil area is limited, and it may easily cause secondary pollution.
The chemical renovation method includes adding a modifier into the soil for adsorption, oxidation, reduction or precipitation, which alters the form of chromium existing in the soil and reduces the bioavailability of chromium. Such a method has the advantages of high chemical reaction speed, short renovation time (e.g., several weeks to several months); high reaction intensity, and low sensitivity to property and concentration of contaminant; and being effective for certain organic substances which can hardly be treated by other methods, thus the method is the most promising technology for treating chromium contaminated soil with practical potential. However, given that the reaction with a reducing agent requires certain conditions, it will frequently produce secondary pollution or destroy the soil structure. For example, the ferrous sulfate reduction method requires an acidic environment of pH 3-4 in order to react with the hexavalent chromium. The common practice in the present relates to adjusting the pH value of the dilute sulfuric acid before the reaction, then using the lime to adjust the pH value to be neutral after the reaction, an introduction of the secondary pollution is inevitable in the process; moreover, the soil granular structure is destroyed, and the plants cannot be grown on the soil after the restoration process. There is not a practical and effective reduction modifier at present.